Such devices are known especially in the form of injection molding machines for thermoplastic plastics. A polymer, usually occurring in the granular form, optionally in conjunction with additives, such as fillers, reinforcing fibers, etc., is charged here into the extruder, often called “injection unit,” and plasticized in the extruder. To inject the plasticized polymer into the molding tool, the screw is axially displaced by a distance to the front in the direction of the discharge opening of the extruder, which is usually formed in the form of a nozzle or of a nozzle unit, and a nonreturn valve, which extensively prevents the backflow of the plasticized polymer during the injection and guarantees the most accurate and reproducible dispensing possible of the necessary quantity of polymer, may be arranged in the area of the free end of the screw. Prior-art nonreturn valves have, in general, a locking ring, which is in contact with the inner circumferential wall of the extruder and which usually ensures satisfactory prevention of backflow of the plasticized, usually highly viscous polymer against the feed of the screw back into the extruder.
Processes have been used recently, in which not necessarily only plasticized polymers are charged into the extruder, but also—as an alternative or in addition—liquid polymers, such as especially monomers, dimers or oligomers, which are capable, optionally in the presence of an activator or initiator or catalyst, of undergoing polymerization with one another or with polymers charged additionally. Such a process is known, for example, from EP 1 415 793 A1. The advantage of such a process is especially that, based on the very low viscosity of the polymers used, it is possible to increase the percentage of fillers or filling fibers, because the latter are infiltrated, wetted or impregnated by the liquid prepolymers to a greater extent than is the case with usually highly viscous polymer melts. However, it proved to be disadvantageous, especially in case of the production of moldings of a relatively complex shape, for example, with relatively small or thin mold cavity areas, into which the polymers and/or prepolymers must be injected, that accurate and especially reproducible dispensing of the desired quantity of polymer or prepolymer is difficult if not outright impossible, which leads to inhomogeneities of the material in the molding and to a resulting increase in the amount of rejects.
US 2005/0233020 A1 describes a nonreturn valve for being arranged at the free end of an extruder of a device of this class for producing plastic moldings. The nonreturn valve may have in this case a locking ring, which is provided on its outer circumferential surface with a circumferential groove in order to receive a piston ring formed from two ring halves, which ensures sealing of the locking ring against the extruder housing, in order to extensively prevent melt from flowing back during the injection operation. Satisfactory tightness, as it would be necessary especially for resins to be processed, whose viscosity ranges from a relatively low viscosity to liquid, and/or for moldings to be produced with a comparatively complex viscosity, cannot, however, be achieved hereby, because, on the one hand, there is a risk of backflow of the resin mixture being injected into the extruder at the junction areas of the piston ring at the very high pressure prevailing during the injection operation in the outlet-side end of the extruder located downstream of the nonreturn valve, and, on the other hand, just like the other elements of the nonreturn valve, the piston ring obviously consists of metal, so that certain tolerances must be necessarily maintained between the piston ring and the extruder in order to ensure the necessary axial displaceability of the locking ring of the nonreturn valve, which said locking ring is provided with the piston ring.
JP 05-042 568 A describes another nonreturn valve for being arranged at the free end of the extruder screw of an injection molding device, wherein an outside coating of the locking ring is provided to seal the locking ring. The coating is formed from diamond-like carbon modifications. Thus, the statements made above in connection with US 2005/0233020 A1 largely apply to the very hard sealing material in the form of diamond-like carbon.